?DESCRIPTION (provided by applicant): The primary cilium is a solitary organelle that is formed when the mother centriole of an interphase cell migrates to the cell surface and pushes out an extension of the plasma membrane. This highly condensed, non-motile structure (present on nearly all cells, including neurons) is the central player in an expanding class of diseases known as ciliopathies. Mutations that cause structural or functional defects in the cilium give rise to a range of clinical disorders that include orofaciodigital, Bardet-Biedl, Meckel and Jouber syndromes, nephronopthisis, and polycystic kidney diseases. Although these are distinct disease entities, many ciliopathies share overlapping phenotypic features, such as severe craniofacial malformations, kidney and liver cysts, polydactyly, obesity, and neurological, sensory, and cognitive impairment at birth. Components of signaling pathways prominent in development (e.g. Hedgehog, Wnt, planar cell polarity) use the primary cilium as their home base. However, it's now apparent that this tiny organelle also harbors many of the constituents of the cyclic AMP (cAMP) signaling circuit, and there is strong, albeit indirect, evidence that thi classical second messenger can intersect with, and modify, other pathways and proteins within the primary cilium. In this Exploratory/Developmental project we will consider the hypothesis that the primary cilium represents a distinct subcellular cAMP / protein kinase A (PKA) signaling compartment. In order to overcome the challenges of working with this very small organelle, we have assembled a panel of optical indicators and genetically encoded tools to monitor and tune the cAMP/PKA signal within the ciliary matrix. In our first Specific Aim, we propose to refine and expand these tools and assess their efficacy in selectively enhancing or inhibiting cAMP signaling and PKA activity in the primary cilium. We will further test their ability to modify the phosphorylation of putative PKA target proteins resident within the cilium. In our second Specific Aim, as proof of concept of the utility of our toolbox, we propose to investigate how intraciliary cAMP affects three biological processes that are known to be in some way linked to both the cAMP pathway and to the primary cilium: (i) the process of ciliary elongation; (ii) cilium-dependent changes in arborization of neuronal dendrites; and (iii) modulation of the Hedgehog signaling pathway.